1. Field of the Invention
The present invention relates to a signal processing device, and, more particularly, to a signal processing device for processing image signals in which total number of horizontal scanning lines forming one screen is predetermined.
2. Related Art
Conventionally, image signals for television or the like (interlace type signals) include those of the NTSC (National Television System Committee) system, the PAL (Phase Alternation by Line) system, and the SECOM (Sequential Colours A Memoirs) system. Among them, for example, image signals for one screen (one frame) in the NTSC system are composed of 525 horizontal scanning lines. When the image signals for one screen in the NTSC system are displayed on the television receiver or the like, it is arranged to divide the image signals into an odd-numbered field (hereinafter called the "Odd field") consisting of 262.5 odd-numbered horizontal scanning lines, and an even-numbered field (hereinafter called the "EVEN field") consisting of 262.5 even-numbered horizontal scanning lines, to alternately display the Odd field and the EVEN field, and to insert the scanning lines of the EVEN field between the scanning lines of the Odd field so that one screen is formed by both fields.
On the other hand, a liquid crystal display device generally used is a one having a display screen which has 480 horizontal scanning lines with 640 dots per line (VGA mode), a TFT (Thin Film Transistors) being disposed as a switching element on each picture element (active matrix driving system). Because such liquid crystal display is designed to be scanned in the non-interlace mode, when it is intended to display the above-mentioned image signals on the liquid crystal display, the image signals which is interlace signals should be converted into non-interlace signals.
Accordingly, when image signals, for example, image signals of the NTSC system are intended to be displayed on the above-mentioned liquid crystal display, because the Odd and EVEN fields of the image signals have 241.5 lines of valid data, respectively, total 1.5 lines at the top and bottom of them are made a non-display area by overscanning, and the image signals in the Odd and EVEN fields for remaining 240 lines are converted into 480 lines of non-interlace signals, respectively.
On the other hand, image signals in the PAL or SECOM system have total scanning lines of 625 lines in one frame, which are 100 lines more than those in the image signals in the NTSC system so that, when the image signals in the PAL or SECOM system are displayed on the liquid crystal display attaining display in the VGA mode for the NTSC system, it is necessary to thin out the scanning lines.
When the image signals in the PAL or SECOM system are displayed on the liquid crystal display attaining display in the VGA mode for the NTSC system, first, to convert the image signals which are interlace type signals into non-interface type signals, the image signals in the Odd and EVEN fields are written in a frame memory to form one frame. Then, when the image signals are read from the frame memory, the scanning lines are thinned out.
However, a memory space of about 1M-bytes becomes necessary to write the image signals in the Odd and EVEN fields in the frame memory so that the device becomes complicated.
Alternatively, two line memories in each of which is image signals for one line are written are used to write image signals for one line in one of them instead of the frame memory. They are read at a speed twice that of writing, while image signals for another one line are written in the other line memory. Thus, display of two lines is performed with the image signals for one line. In this way, when the image signals of the PAL or SECOM system is displayed on the liquid crystal display for the NTSC system, write or read of signals for one line should be stopped for thinning. Therefore, the image signals cannot be properly displayed.
The present invention is invented in view of the above facts, and intended to provide a signal processing device which converts interlace image signals into those for the non-interlace mode with a simple arrangement, and which can thin out image signals with a simple arrangement.